In the past, it has been used a PZT (PbTiO3—PbZrO3) component-based ceramic containing lead as a piezoelectric ceramic composition. The reason for it is that the aforesaid PZT exhibits excellent piezoelectric properties and a high mechanical quality factor, and it is easy to produce materials exhibiting various desired characteristics which are required for various applications such as a sensor, an actuator, and a filter. Further, since the PZT has a high relative dielectric constant, it may also be used as a capacitor.
However, while the piezoelectric ceramic composition composed of the aforesaid PZT exhibits excellent characteristics, it contains lead as an element constituting the composition, leading to a concern that harmful lead may be eluted from industrial waste of products containing PZT to result in an environmental pollution. The increasing awareness of environmental concern in recent years has made it difficult to manufacture the products containing substances such as PZT which may be a cause of environmental pollution.
In order to resolve the above-described problem caused by the inclusion of lead as a component, there has been required a new piezoelectric ceramic composition containing no lead. In this situation, a piezoelectric ceramic composition having KNbO3—NaNbO3 as a main composition has been paid attention since it contains no lead and exhibits excellent piezoelectric properties.
However, although the above-described piezoelectric ceramic composition having KNbO3—NaNbO3 as a main composition exhibits a high Curie temperature (the first order phase transition) such as about 200° C. or higher, there exists the second order phase transition which is formed by a phase change from a ferroelectric phase at a lower temperature to a ferroelectric phase at a higher temperature in the temperature range of near room temperature. As a result, there is a possibility that degradation of properties may occur to produce a problem for practical use when the temperature cycle passes through the second order phase transition. Therefore, many technologies were proposed to decrease the temperature of the second order phase transition to be below the room temperature.
In Patent document 1, there was disclosed a piezoelectric ceramic composition containing potassium sodium lithium niobate, strontium titanate and bismuth iron oxide and represented by the formula of (1−a−b)(KxNayLi1-x-y)NbO3+aSrTiO3+bBiFeO3, 0<a≦0.1, 0<b≦0.1, 0<x≦0.18, 0.8<y<1, provided that potassium sodium lithium niobate is (KxNayLi1-x-y)NbO3, strontium titanate is SrTiO3, and bismuth iron oxide is BiFeO3.
According to Patent document 1, the piezoelectric ceramic composition composed of potassium sodium lithium niobate as a maim component can be controlled as follows. The composition of which is made rich in Na and strontium titanate is incorporated in an appropriate amount in the compound, and further an appropriate amount of rhombohedral crystal BiFeO3 is include in the compound to become Curie temperature of about 870° C. As a result of multiple solid solute of different crystals to (NaKLi)NbO3, the phase transition temperature can be changed, thereby it can be decreased the temperature change of elastic constant and the discontinuously changing portion of piezoelectric d33 constant at around room temperature.
In Patent document 2, it was disclosed a piezoelectric ceramic composition made of a perovskite compound which is composed of Na, K, Li, Sr, Nb, and Ti as a main component, in which the formula in molar ratio is represented as: (1−x−y)KNbO3+xNaNbO3+y(LiNbO3+SrTiO3), provided that x and y satisfy 0.42≦x≦0.50, 0.06≦y≦0.12.
According to Patent document 2, it is possible to decrease the second order phase transition temperature of the aforesaid piezoelectric ceramic composition to be below the room temperature. This can be achieved with the 4 component composition which was made by substituting a part of the composition of the 2 component piezoelectric ceramic composition mainly composed of KNbO3—NaNbO3 with LiNbO3 and SrTiO3 at the same time.    Patent document 1: JP-A No. 2007-145650    Patent document 2: JP-A No. 2007-55867